1. Field of the Invention
The present invention generally relates to an acoustic surface wave device, and more particularly to an acoustic surface wave device suitably used in a high-frequency circuit.
2. Description of the Prior Art
With development of a small, light weight car telephone or portable telephone, filters using an acoustic surface wave have been developed to be incorporated in the car telephone or the portable telephone. Since an electrical circuit used in the car telephone or the portable telephone is an high-frequency circuit, the acoustic surface wave device is required to be suitable for the high-frequency wave.
FIGS. 1 and 2 show a conventional acoustic surface wave device 100. The acoustic surface wave device 100 is provided in the high-frequency circuit provided in, for example, the car telephone or the portable telephone, as an filter. FIG. 1 is an exploded perspective view, FIG. 2A is a perspective view and FIG. 2B is a perspective bottom view.
The acoustic surface wave device 100 includes a package 102, an acoustic surface wave element 103 and a metallic cap 104. The package 102 includes first, second and third ceramic substrates 105, 106, 107 provided in a stacked formation. In the package 102, an input terminal 108 and an output terminal 109, which are dotted in the drawing, are provided.
The input terminal 108 has a pair of input ground terminals 111, 112 and an input signal terminal 110 provided between the input ground terminals 111, 112. The output terminal 109 has a pair of output ground terminals 114, 115 and an output signal terminal 113 provided between the output ground terminals 114, 115.
The first ceramic substrate 105 is provided at the bottom of the package 102. On the first ceramic substrate 105, a die-attach portion 116 (shown as a dotted portion) is provided. The acoustic surface wave element 103 is mounted on the die-attach portion 116. The die-attach portion 116 has input ground connecting portions 118, 119, which are electrically connected to the input ground terminals 111, 112. Also, the die-attach portion 116 has output ground connecting portions 120, 121, which are electrically connected to the output ground terminals 114, 115. The input ground connecting portions 118, 119 and the output ground connecting portions 120, 121 are formed integrally with the die-attach portion 116.
The input signal terminal 110, the input ground terminals 111, 112, the output signal terminal 113 and the output ground terminals 114, 115 are exposed from the bottom face of the substrate body 105a to form foot patterns 110a-115a functioning as outer connecting terminals.
The second ceramic substrate 106 is provided in the middle of the package 102. The second ceramic substrate 106 includes a substrate body 106a and a plurality of pads 127-132 of electrical conductive metal layers formed on the substrate body 106a. The pads 127-132 are connected to the terminals 110-115 facing the pads 127-132. The pads 127-132 are electrically connected to the acoustic surface wave element 103 through wires (not shown). In a central portion of the substrate body 106a, an opening 133 housing the acoustic surface wave element 103 is formed. At four corners of the substrate body 106a of the second substrate 106, notches 134a-134d are formed.
The third ceramic substrate 107 is provided at the top of the package 102. In a central portion of the third ceramic substrate 107, an opening 136 is formed, which is larger than the opening 133 formed on the second ceramic substrate 106. On the whole surface of the third ceramic substrate 107, an upper wiring layer 137 (dotted portion) is formed to be electrically connected to the metallic cap 104. A substrate body 107a has four notches 138a-138d provided at each corner thereof, in which cap connecting wires 139a-139d (dotted portion) are provided. The cap connecting wires 139a-139d are electrically connected to the upper wiring layer 37.
The first through third ceramic substrates 105, 106, 107 are joined to form the package 102 shown in FIGS. 2A and 2B. After the package 102 is formed, the input signal terminal 110 and the input ground terminals 111, 112, the output signal terminal 113, the output ground terminals 114, 115 are electrically connected to form the input terminal 108 and the output terminal 109. Also, the cap connecting portions 140a-140d formed at each corner of the second ceramic substrate 106 are electrically connected to the cap connecting wires 139a-139d formed in the notches 138a-138d of the third ceramic substrate 107.
The acoustic surface wave element 103 is mounted in the die-attach portion 116 formed on the first ceramic substrate 105. An electrode provided on the acoustic surface wave element 103 is connected to the pads 127-132 provided on the second ceramic substrate 106 through wires. The acoustic surface wave element 103 is electrically connected to the input terminal 108 and the output terminal 109.
The metallic cap 104 is joined to the upper wiring layer 137 covering the opening 136 formed in the third ceramic substrate 107. The metallic cap 104 can be joined to the upper wiring layer 137 through a solder using a gold (Au)-tin (Sn) alloy or a tin (Sn)-lead (Pb) alloy. Thus, the metallic cap 104 is electrically connected to the input or output ground terminals 111, 112, 114, 115.
That is, the metallic cap 104 is electrically connected to each of the input or output ground terminals 111, 112, 114, 115 through the upper wiring layer 137, the cap connecting wires 139a-139d, the cap connecting portions 140a-140d, the ground pads 128, 129, 131, 132. Thus, the metallic cap 104 has a ground electric potential and functions as shielding characteristics to the acoustic surface wave element 103.
FIG. 3 is a circuit diagram of the package 102 of the conventional acoustic surface wave device 100. In the acoustic surface wave device 100 using the high-frequency wave, the signal terminals 110, 113 are electrically connected to the ground terminals 111, 112, 114, 115 in the package 102. Also, since the ground terminals 111, 112, 114, 115 have an impedance, the ground terminals 111, 112, 114, 115 cannot have an ideal ground electric potential. Also, the metallic cap 104 having the shielding effect and the die-attach portion cannot have an ideal ground electric potential.
Z1-Z14 in FIG. 3 represent an impedance of various portions in the package 102. Each represents the impedance as follows.
Z1: Impedance of the input signal terminal 110 PA1 Z2: Impedance due to a capacitance between the input signal pad 127 and the ground pads 128, 129 PA1 Z3: Impedance of the input ground terminals 111, 112 PA1 Z4: Impedance due to a capacitance between the input signal foot pattern 110a and the ground foot patterns 111a, 112a PA1 Z5: Impedance of the output signal terminal 113 PA1 Z6: Impedance due to a capacitance between the output signal foot pattern 113a and the ground foot patterns 114a, 115a PA1 Z7: Impedance of the output ground terminals 114, 115 PA1 Z8: Impedance due to a capacitance between the output signal pad 130 and the ground pads 131, 132 PA1 Z9: Total impedance of the input cap connecting portions 139a, 139b and the upper wiring layer 137 PA1 Z10: Impedance of the metallic cap 104 PA1 Z11: Total impedance of the output cap connecting portions 139c, 139d and the upper wiring layer 137 PA1 Z12: Impedance of the input ground connecting portions 118, 119 PA1 Z13: Impedance of the die-attach portion 116 PA1 Z14: Impedance of the output ground connecting portions 120, 121
In the conventional acoustic surface wave device 100, the input ground terminals 111, 112 are electrically connected to the output ground terminals 114, 115 through Z9-Z14. That is, the input ground terminals 111, 112 are electrically connected to the output ground terminals 114, 115 through the metallic cap 104 and the die-attach portion 116.
In the acoustic surface wave device 100 in which the input ground terminals 111, 112 are electrically connected to the output ground terminals 114, 115, the attenuation characteristics thereof are deteriorated by the package 102, especially when a high-frequency wave is handled.